Many of the red-emitting phosphors are derived from silicon nitride (Si3N4). The structure of silicon nitride comprises layers of Si and N bonded in a framework of slightly distorted SiN4 tetrahedra. The SiN4 tetrahedra are joined by a sharing of nitrogen corners such that each nitrogen is common to three tetrahedra. See, for example, S. Hampshire in “Silicon nitride ceramics—review of structure, processing, and properties,” Journal of Achievements in Materials and Manufacturing Engineering, Volume 24, Issue 1, September (2007), pp. 43-50. Compositions of red-emitting phosphors based on silicon nitride often involve substitution of the Si at the center of the SiN4 tetrahedra by elements such as Al; this is done primarily to modify the optical properties of the phosphors, such as the intensity of the emission, and the peak emission wavelength.
There is a consequence of the aluminum substitution, however, which is that since Si4+ is being replaced by Al3+, the substituted compound has a missing positive charge. Since nature does not allow unbalanced electrical charges in materials, this missing positive charge has to be balanced in some manner. An approach used to achieve charge balance requires the Al3+ for Si4+ substitution to be accompanied by a substitution of O2− for N3−, such that the missing positive charge is counter-balanced with a missing negative charge. This leads to a network of tetrahedra that have either Al3+ or Si4+ as the cations at the centers of the tetrahedra, and a structure whereby either an O2− or an N3− anion is at the corners of the tetrahedra. Since it is not known precisely which tetrahedra have which substitutions, the nomenclature used to describe this situation is (Al,Si)3—(N,O)4. Clearly, for charge balance there is one O for N substitution for each Al for Si substitution.
Furthermore, these substitutional mechanisms for charge balance—O for N—may be employed in conjunction with an interstitial insertion of a cation. In other words, the cation is inserted between atoms preexisting on crystal lattice sites, into “naturally occurring” holes, interstices, or channels. For example, the use of cations in Sr-containing α-SiAlONs have been discussed by K. Shioi et al. in “Synthesis, crystal structure, and photoluminescence of Sr-α-SiAlON:Eu2+,” J. Am. Ceram Soc., 93 [2] 465-469 (2010). Shioi et al. give the formula for the overall composition of this class of phosphors: Mm/vSi12-m-nAlm+nOnN16-n:Eu2+, where M is a cation such as Li, Mg, Ca, Y, and rare earths (excluding La, Ce, Pr, and Eu), and v is the valence of the M cation. As taught by Shioi et al., the crystal structure of an α-SiAlON is derived from the compound α-Si3N4. To generate an α-SiAlON from α-Si3N4, a partial replacement of Si4+ ions by Al3+ ions takes place, and to compensate for the charge imbalance created by Al3+ substituting for Si4+, some O substitutes N and some positive charges are added (what Shioi et al. refer to as “stabilization”) by trapping the M cations into the interstices within the network of (Si,Al)—(O,N)4 tetrahedra.
The discovery of the europium doped alkaline earth metal silicon nitride phosphor (M2Si5N8 where M is Ca, Sr, or Ba) was made in 2000 by several groups, one of which produced the PhD thesis by J. W. H. van Krevel at the Technical University Eindhoven, January 2000, Osram Opto Semiconductors U.S. Pat. No. 6,649,946, and H. A. Hoppe, et al., J. Phys. Chem. Solids. 2000, 61:2001-2006. This family of phosphors emits at wavelengths from 600 nm to 650 nm with high quantum efficiency. Among them, pure Sr2Si5N8 had the highest quantum efficiency and emitted at a peak wavelength of about 620 nm. It is well known that this red nitride phosphor has poor stability under the conditions wherein the LED is operated at a temperature ranging from 60 to 120° C. and an ambient humidity ranging from 40 to 90%.
U.S. Pat. No. 8,076,847 (the '847 patent) to Tamaki et al. is directed to a nitride phosphor represented by the general formula LxMyN(2/3)X+(4/3)Y):R or LXMYOZN((2/3)X+(4/3)Y-(2/3)Z):R, wherein L is at least one or more selected from the Group II elements consisting of Mg, Ca, Sr, Ba and Zn, M is at least one or more selected from the Group IV Elements in which Si is essential among C, Si and Ge, and R is at least one or more selected from the rare earth elements in which Eu is essential among Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Lu. Various embodiments of the patent further contain a Group I[A] element consisting of Li, Na, K, Rb, and Cs. This Group I[A] element functions as a flux during synthesis and not as an interstitial cation for charge balance, the purpose of the flux being to control the particle diameter.
US 2010/0288972 to Roesler et al. teach a AE2Si5N8:RE phosphor with a charge compensation mechanism that counters a charge imbalance caused by oxygen, whether introduced intentionally or non-intentionally. If the oxygen is introduced non-intentionally then it is a contamination, but in either event, the charge imbalance caused by the replacement of nitrogen (N3−) with oxygen (O2−) has to be balanced. Roesler et al. accomplish this in US 2010/0288972 either by the substitutional replacement of silicon (Si4+) with a column IIIB element, such as aluminum (Al3+), or by the replacement of the alkaline earth content originally present in the phosphor (e.g., Sr2+ or Ca2+) by an alkali metal (such as Li+, Na+, or K+). Note that the authors reported no shift in the peak emission wavelength as a result of the substitutions and charge balancing, and further, no enhanced stability was reported with accelerated environmental stress testing (e.g., temperature and humidity aging).
The forms of charge compensation reported in the art are not believed to render the phosphor more impervious to thermal/humidity aging, nor do they appear to accomplish the beneficial result of increasing the peak emission wavelength with little or substantially no alteration of photoemission intensity.
There is a need for stabilized silicon nitride-based phosphors and stabilized M2Si5N8-based phosphors with: peak emission wavelengths over a wider range in the red and also other colors; and with enhanced physical properties of the phosphor, such as temperature and humidity stability.